Flues can first be divided into open flues and closed flues. Open flues are also known as Conventional Flues (CF). Closed flues are better known as Balanced Flues (BF) or Fanned Flues (FF), and are used with Room Sealed (RS) appliances.

Open (conventional) flues.

An open flued appliance draws combustion air from its surroundings. Ventilation to ensure that this air can pass from outside to the room containing the appliance is vitally important. Regulations specify what ventilation should be provided in each case.

The open flue usually incorporates a draft diverter near its base. The section of flue from the appliance heat exchanger up to the draft diverter is the primary flue, and that after the draft diverter is the secondary flue. Since draught diverters are usually incorporated into the appliance, the primary flue is only a short section from the top side of the heat exchanger to the draught diverter.

The draught diverter provides a break (air gap) between the appliance and the main (secondary) flue. This is vital because the "pull" or suction exerted by the main flue will vary considerably with height and configuration, yet the appliance needs to operate within it's design pull which is provided by the built-in components including the primary flue.

The other important function of the draught diverter (as its name suggests) is to allow the occasional down-draught (caused by sudden gusts of wind) to pass into the room without disturbing the combustion in the appliance.

Normally the draught diverter will pull in additional air as well as the Products Of Combustion (POCs) from the boiler. This can be tested by using a smoke match (or any small flame) placed below and to the edge of the opening of the draught diverter. The smoke (or flame) will be drawn across, demonstrating the pull.

The open flue continues vertically upwards, with some lateral deviations allowable, to a terminal in open air. The flue can run internally, externally or within an existing chimney (using a flexible flue liner). The greater the vertical height of the flue, the better the pull. However lateral deviations add to the resistance and reduce the pull.

Balanced flues.

Room sealed appliances have either a balanced flue or a fanned flue. Both will include an air duct to bring combustion air to the appliance, so there is no air movement to or from the room containing the appliance. This makes Room Sealed appliances intrinsically safe, since products of combustion (POCs) cannot normally escape into the room.

The balanced flue terminal has the flue outlet and air inlet in close proximity, so that any draughts or gusts of wind affect both equally and cancel out in the appliance. In this way the burner flame and even pilot lights are not affected.

Because balanced flues are only powered by natural draught (the draught created by the hot flue gases), they must be kept quite short, usually just sufficiently long to pass through the external wall against which the appliance must be installed.

Fanned flues.

Fanned flues use the pressure created by a fan to power the movement of outside air into the appliance and combustion products to external air. This allows both the air ducts and flue ducts to be much longer, of smaller diameter, and to include changes of direction. They can run and terminate horizontally or vertically, or a combination of the two.

In most cases Fanned Flues are concentric, meaning the air duct contains the smaller flue duct within it. This has the advantage that a leak or break in the inner flue duct will only leak POCs back into the boiler rather than into a room. However even longer runs are possible with Two pipe systems where the flue and air ducts run separately.

Some fanned flues are also open flues, taking combustion air from the room housing the appliance. As with all open flues, The provision of adequate ventilation to supply combustion air to such appliances is vitally important.

Terminals.

In all cases (open, balanced and fanned) the flue terminal (and air inlet) must be located according to various rules and standards to ensure that Products Of Combustion (POCs) can disperse safely and to avoid down draughts.

Any comments on this post can be posted below. The post has already been modified to take account of comments made.

Chris, that's excellent. It clears up pretty well all the queries I had about previous definitions.

The penny has dropped. You seem to be treating "the flue" as one system, which in many respects it is, whereas most of us ordinary mortals think of it as the bit that lets out the stink. It's like how a car mechanic views the exhaust and air intake as two different systems, which in many respects they are.

You may now understand where the comprehension gap lay.

"The open flue usually incorporates a draft diverter at its base. The section of open flue before the draft diverter is the primary flue, and that after the draft diverter the secondary flue...."

"...The open flue continues vertically upwards".

What's needed is mention of the fact that there's a stonking great boiler in between the two! Come to that, it's not clear whether the draught diverter - whatever that is or does - is before or after the heat source. See the problem?

On style, I'd suggest capitalisation is reserved for formal expressions that may be referred to later by initials or acronym. Combustion Products and Concentric are outside this scope, although you do talk about Products of Combustion (POC).

You are right that the flue system and air supply should be considered as a whole, but I'm afraid you've misunderstood exactly what I meant (or I didn't make it clear enough - in the context of DIY advice the onus is on the explainer to be clear).

The primary flue is not the air duct. It is a very short section of flue from the top side of the heat exchanger to the draught diverter. In most boilers it is incorporated into the boiler and so short that you would hardly notice it, but nevertheless it is technically important.

I have modified the OP to include a better explanation of this. I take your other points which I will incorporate into my post above and others.